Lightweight shrouded fan

ABSTRACT

A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, a fan section including a fan with a plurality of fan blades rotatable about an axis and a shroud and a speed change device in communication with the fan.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressor section, a combustor section, and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section.

The fan section includes multiple airfoils disposed circumferentially about an engine longitudinal centerline axis. At certain aircraft operating conditions, these airfoils may experience self-induced oscillations, such as flutter. These self-induced oscillations may become severe enough to fracture the airfoil. One means of preventing such a fracture is to increase the chord width of the fan blades. However, this approach increases the overall weight of the engine and the rotating mass. Accordingly, it is desirable to develop an improved gas turbine engine design that will reduce flutter of the airfoils and decrease the weight of the engine.

SUMMARY

A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, a fan section including a fan with a plurality of fan blades rotatable about an axis and a shroud and a speed change device in communication with the fan.

In a further non-limiting embodiment of the foregoing gas turbine engine, the speed change device includes a geared architecture driven by a turbine section for rotating the fan about the axis.

In a further non-limiting embodiment of either of the foregoing gas turbine engines, the shroud is located at radially outer ends of the plurality of fan blades.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the gas turbine engine includes a low pressure turbine with at least three stages and no more than six stages.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the gas turbine engine includes a fixed area nozzle in communication with the fan section.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the gas turbine engine includes a variable area nozzle in communication with the fan section.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the gas turbine engine includes a seal between the shroud and a fan case.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the seal includes at least one knife edge.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the seal includes a honeycomb structure.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the seal includes a rubber member on a radially inner surface of the fan case.

In a further non-limiting embodiment of any of the foregoing gas turbine engines, the shroud is made of a fiber composite.

A method of assembling a gas turbine engine according to another exemplary aspect of the present disclosure includes, among other things, positioning a turbine section in communication with a shaft. A speed change device is positioned in communication with the shaft. A fan section is positioned in communication with the speed change device. The fan section includes a plurality of fan blades and a shroud.

In a further non-limiting embodiment of the foregoing method of assembling a gas turbine engine, the speed change device includes a geared architecture.

In a further non-limiting embodiment of either of the foregoing methods of assembling a gas turbine engine, the shroud is located at the radially outer ends of the plurality of fan blades.

In a further non-limiting embodiment of any of the foregoing methods of assembling a gas turbine engine, the turbine section is a low pressure turbine.

In a further non-limiting embodiment of any of the foregoing methods of assembling a gas turbine engine, the turbine section includes at least three stages and no more than six stages.

In a further non-limiting embodiment of any of the foregoing methods of assembling a gas turbine engine, a fixed area fan nozzle is positioned in communication with the fan section.

A method of operating a gas turbine engine according to another exemplary aspect of the present disclosure includes, among other things, rotating a fan section including a shroud at a first speed and rotating a turbine section at a second speed. The first speed is different from the second speed.

In a further non-limiting embodiment of the foregoing method of operating a gas turbine engine, a speed change device is in mechanical communication with the fan section and the turbine section.

In a further non-limiting embodiment of the foregoing method of operating a gas turbine engine, the turbine section is a low pressure turbine.

Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples.

These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example gas turbine engine.

FIG. 2 is a perspective view of a shrouded fan.

FIG. 3 is a partial cross-sectional view of a fixed area fan nozzle.

FIG. 4 is a partial cross-sectional view of a variable area fan nozzle.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example gas turbine engine 20 that includes a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B while the compressor section 24 draws air in along a core flow path C where air is compressed and communicated to a combustor section 26. In the combustor section 26, air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through the turbine section 28 where energy is extracted and utilized to drive the fan section 22 and the compressor section 24.

Although the disclosed non-limiting embodiment depicts a turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section.

The example engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that connects a fan 62 and a low pressure (or first) compressor section 44 to a low pressure (or first) turbine section 46. The inner shaft 40 drives the fan 62 through a speed change device, such as a geared architecture 48, to drive the fan 62 at a lower speed than the low speed spool 30. The high-speed spool 32 includes an outer shaft 50 that interconnects a high pressure (or second) compressor section 52 and a high pressure (or second) turbine section 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate via the bearing systems 38 about the engine central longitudinal axis A.

A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. In one example, the high pressure turbine 54 includes at least two stages to provide a double stage high pressure turbine 54. In another example, the high pressure turbine 54 includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine.

The example low pressure turbine 46 has a pressure ratio that is greater than about 5. The pressure ratio of the example low pressure turbine 46 is measured prior to an inlet of the low pressure turbine 46 as related to the pressure measured at the outlet of the low pressure turbine 46 prior to an exhaust nozzle. In one non-limiting embodiment, the low pressure turbine 46 includes at least three stages and no more than 6 stages. In another non-limiting embodiment, the low pressure turbine 46 includes at least three stages and no more than 4 stages.

A mid-turbine frame 58 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 58 further supports bearing systems 38 in the turbine section 28 as well as setting airflow entering the low pressure turbine 46.

Air flowing through the core flow path C is compressed by the low pressure compressor 44 then by the high pressure compressor 52 mixed with fuel and ignited in the combustor 56 to produce high speed exhaust gases that are then expanded through the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 58 includes vanes 60, which are in the core airflow path and function as an inlet guide vane for the low pressure turbine 46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guide vane for low pressure turbine 46 decreases the length of the low pressure turbine 46 without increasing the axial length of the mid-turbine frame 58. Reducing or eliminating the number of vanes in the low pressure turbine 46 shortens the axial length of the turbine section 28. Thus, the compactness of the gas turbine engine 20 is increased and a higher power density may be achieved.

The disclosed gas turbine engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the gas turbine engine 20 includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example geared architecture 48 is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 2.3.

In one disclosed embodiment, the gas turbine engine 20 includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of the low pressure compressor 44. It should be understood, however, that the above parameters are only exemplary of one embodiment of a gas turbine engine including a geared architecture and that the present disclosure is applicable to other gas turbine engines.

A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft., with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption ('TSFC')”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point.

“Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45.

“Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tip speed”, as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second.

The example gas turbine engine includes the fan 62 that comprises in one non-limiting embodiment less than about 26 fan blades 42. In another non-limiting embodiment, the fan section 22 includes less than about 20 fan blades. Moreover, in one disclosed embodiment the low pressure turbine 46 includes no more than about 6 turbine rotors schematically indicated at 34. In another non-limiting example embodiment the low pressure turbine 46 includes about 3 turbine rotors. A ratio between the number of fan blades 42 and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine 46 provides the driving power to rotate the fan section 22 and therefore the relationship between the number of turbine rotors 34 in the low pressure turbine 46 and the number of blades 42 in the fan section 22 disclose an example gas turbine engine 20 with increased power transfer efficiency.

The example gas turbine engine 20 includes fan blades 42 that extend from a central disk 64 on a radially inner end to a shroud 66 on a radially outer end. In this example, the shroud 66 is comprised of a fiber composite, such as a woven fiber composite or a wound fiber composite. Due to the geared architecture 48, the fan 62 rotates at a slower speed than the low pressure turbine 46. Because the fan 62 has a lower rotational speed, the fan blade tip velocity decreases and the aerodynamic losses that would normally accompany a shrouded fan are reduced. The gas turbine engine 20 generates a similar amount of thrust as a gas turbine engine with a fan section that rotates at the same speed as the low pressure turbine by increasing the length and number of fan blades 42. The fan 62 accommodates more fan blades 42 by decreasing the chord width of the fan blades 42 to allow for more fan blades 42. Increasing the length and number of fan blades 42 and decreasing the chord width of the individual fan blades 42 will decrease the overall weight of the gas turbine engine 20 as well as the rotating mass.

The fan section 22 includes an example fan case 74. An outer surface 68 of the shroud 66 includes at least one knife edge 70 that form a seal between the outer surface 68 of the shroud 66 and an inner surface 72 of the fan case 74 to prevent air leakage between the shroud 66 and the fan case 74. In this example, the at least one knife edge 70 includes three knife edges that extend around the outer surface 68 of the shroud 66 (FIG. 2). The inner surface 72 of the fan case 74 includes an outer air seal 76, such as a rubber member, to prevent air leakage between the shroud 66 and the fan case 74. The outer air seal 76 accommodates for expansion of the fan 62 without causing damage to the fan case 74 because of the elastic nature of the outer air seal 76.

In one non-limiting embodiment, the shroud 66 includes a honeycomb structure 78 on the outer surface 68 for forming an additional seal between the shroud 66 and the fan case 74 by creating a disturbance in the air flowing between the shroud 66 and the fan case 74.

In one non-limiting embodiment, the fan nozzle 65 includes a fixed area fan nozzle such that the exit area for the fan section 22 is fixed during operation of the gas turbine engine 20 (FIG. 3). Eliminating a variable area fan nozzle from the gas turbine engine 20 provides a significant weight loss over convention gas turbine engines with variable area fan nozzles. A variable area fan nozzle can be eliminated from the gas turbine engine 20 because of the gas turbine engine's 20 ability to prevent flutter through the use of the shroud 66 and the lower rotational speed of the fan 62 due to the geared architecture 48.

In another non-limiting embodiment, the fan nozzle 65′ includes a variable area fan nozzle such that the exit area of the fan section 22 is varied during operation of the gas turbine engine 20 (FIG. 4). Increasing the exit area of the fan section will prevent flutter of the fan blades 42 from occurring by decreasing the pressure downstream of the fan blades 42.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. A gas turbine engine comprising: a fan section including a fan with a plurality of fan blades rotatable about an axis and a shroud; and a speed change device in communication with the fan.
 2. The gas turbine engine of claim 1, wherein the speed change device includes a geared architecture driven by a turbine section for rotating the fan about the axis.
 3. The gas turbine engine of claim 1, wherein the shroud is located at radially outer ends of the plurality of fan blades.
 4. The gas turbine engine of claim 1, including a low pressure turbine with at least three stages and no more than six stages.
 5. The gas turbine engine of claim 1, including a fixed area nozzle in communication with the fan section.
 6. The gas turbine engine of claim 1, including a variable area nozzle in communication with the fan section.
 7. The gas turbine engine of claim 1, including a seal between the shroud and a fan case.
 8. The gas turbine engine of claim 7, wherein the seal includes at least one knife edge.
 9. The gas turbine engine of claim 7, wherein the seal includes a honeycomb structure.
 10. The gas turbine engine of claim 7, wherein the seal includes a rubber member on a radially inner surface of the fan case.
 11. The gas turbine engine of claim 1, wherein the shroud is made of a fiber composite.
 12. A method of assembling a gas turbine engine comprising the steps of: positioning a turbine section in communication with a shaft; positioning a speed change device in communication with the shaft; and positioning a fan section in communication with the speed change device, the fan section includes a plurality of fan blades and a shroud.
 13. The method as recited in claim 12, wherein the speed change device includes a geared architecture.
 14. The method as recited in claim 12, wherein the shroud is located at the radially outer ends of the plurality of fan blades.
 15. The method as recited in claim 13, wherein the turbine section is a low pressure turbine.
 16. The method as recited in claim 14, wherein the turbine section includes at least three stages and no more than six stages.
 17. The method as recited in claim 12, including positioning a fixed area fan nozzle in communication with the fan section.
 18. A method of operating gas turbine engine comprising the steps of: rotating a fan section including a shroud at a first speed; rotating a turbine section at a second speed, wherein the first speed is different from the second speed.
 19. The method as recited in claim 18, wherein a speed change device is in mechanical communication with the fan section and the turbine section.
 20. The method as recited in claim 18, wherein the turbine section is a low pressure turbine. 